The present invention relates to a method and apparatus for focusing light.
There are many instances in which it is desirable to modify the focal length of an optical system having any number of lenses, mirrors or combinations thereof. A common example of a variable focal length optical system is a zoom lens; it can change the focal length of a camera or other optical system over a wide latitude by changing the physical position of different lenses relative to each other. Creating smaller changes in focal length, however, is far more difficult due to the high tolerances required for the optical elements. The focal length of an optical element is typically determined by grinding and polishing the optical elements to an appropriate curvature. Optical elements can have slightly different focal lengths if they have slightly different curvatures. It is well known in the art that an optical surface is relatively easy to polish smooth but that the resulting curvature is very difficult to control. Indeed, the minimum amount by which the focal length can be controlled is limited by how thin a slice of material can be reliably removed from the surface of a lens.
The ability to produce optical elements having focal lengths that differ only a small amount becomes progressively more expensive as the difference in the focal length becomes less. Small differences in focal length require high precision for the surface to match the desired geometric shape. A curved optical surface cannot be approximated beyond the point where material can no longer be removed in sufficiently thin layers. The ultimate limit of matter to approximate any curved surface is the molecular composition of the material; the quantized nature of matter limits the approximation of an optical surface to the molecular layers. Chemistry limits how few molecular layers can be removed at a time from a given material. Removing single molecular layers strains optical fabrication to its ultimate limit. While higher precision optics are desirable for many applications, it is not practical to make high precision optics by mechanical grinding.
The difficulty in forming very slight differences in focal length is best exemplified in a long focal length lens. The f ratio of an optical system is defined by the ratio of the aperture of the system divided by its focal length. Optical systems having a high f ratio are difficult to make due to the extraordinary tolerances required to form the curved surfaces. For example, a simple plano convex lens having a focal length that is 100-200 times the diameter of the lens, i.e. f=100-200, is nearly flat. The curvature needed to obtain this tolerance can require that the thickness of the lens change by only a few molecular layers of glass across its entire aperture. This tolerance is almost impossible to obtain; curved optical surfaces that are so nearly flat are not commercially feasible.
Lenses having greater curvatures can be combined to produce very long focal length optical systems. However, a combination of lenses requires at least two high precision optical surfaces. The tolerances in curvature are impractical even though the optical surfaces have a greater curvature and are therefore easier to make. The resulting optical system is still quite expensive. Very long focal length optical systems tend to be one of a kind systems since it is not feasible to mass produce a large number of highly accurate lenses.
It is known in the art to form optical elements from material having a variable index of refraction so as to change the focal length of an optical system. For example, U.S. Pat. Nos. 4,441,791 and 4,592,628 disclose reflective surfaces that can be deformed electronically. U.S. Pat. Nos. 4,572,616 and 4,601,545 disclose lenses that use liquid crystal material to produce a lens having a variable focal length. Such lenses have been incorporated into eyeglasses as a substitute for traditional bifocal or trifocal lenses as illustrated in U.S. Pat. No. 4,795,248.
A lens made from a material having a dynamic index of refraction could, in principle, change the focal length of a material by any amount. However, known dynamic systems have not been applied to creating small changes in focal length. The refractive surfaces in the above patents probably cannot adjust to very small changes because the electrostatic field across the curved surface cannot be made sufficiently uniform. These nonuniform electrostatic forces should cause the index of refraction of the electro-optic material to change by small amounts across the aperture of the lens. This nonuniformity should be sufficient to distort the curvature of the lens by the same or greater amount required to cause a small change in focal length.
The index of refraction of various materials can also be changed using electrostatic and magnetic fields, temperature, and pressure. It is significant, however, that none of these materials have been applied to producing the small changes in focal length needed to produce a high precision optical system. Electro-optical materials remain the material of choice despite the uneven refractive index that they presumably leave across the aperture of the lens; it seems unlikely that a sufficiently smooth variation can ever be generated using electro-optical material. Indeed, none of the optical systems set forth in the foregoing references are high precision optics.
There exists a need in the art for a high precision optic that can change the focal length of an optical system by very small amounts without having to mechanically grind and polish a lens. It would be even better if the change in the focal length were dynamic so that the focal length could change. It would help to have an optical element that could change the focal length of an optical system by an amount equal to or less than a single molecular layer of material so that the change in focal length is no longer limited by the chemical and molecular properties of matter from which the lens is made. An appropriate optical element would have immediate application for forming high precision lenses, very long focal length optical systems and for changing the focal lengths of optical systems by very small amounts.